The purpose of these studies is to obtain further information with regard to the metabolism of the cardiac glycosides. In particular, these studies have focused upon the determinants of the myocardial uptake of the digoxin as affected by serum electrolytes. Most recently, studies have been carried out to determine the effect of concomitant glucose and insulin administration on the metabolism of digoxin. These investigations are now being extended to obtain the relevance of experiments in animals to similar situations in man. The importance of these investigations are that it is now possible to measure blood concentrations of the digitalis compound. It is important to understand the meaning of these blood concentrations and their relation to the myocardial concentration of the drug.